Great question! If you have too many links and need to start a second row, just click the Effects tab in the properties editor, and look/scroll down to the bottom to where it says Horizontal Wrap. This number controls how many links you have horizontally. If the number of links in your menu exceeds this number then it will automatically start another row. So, in your case, you may want to drop it down to about 7 or 8, then create whatever new links you need. This way they won't be so bunched up on a single row.

Which site are you working on? If it is a new Websitebuilder site, then it will be different, but you still click on the menu, and then on more and then on advanced to see the properties editor on the right. This will give you the complete array of options for the menu.